fix: make role optional in messages

[MadaraUchiha-314]

• Makes the role attribute in Message optional
• Many types included in AnyMessage don't have role in it. This makes it difficult for anyone implementing the protocol to send a response back as there's validation failure when AnyMessage is converted to Message

[nfcampos]

If a message has no role how do you distinguish it from other message types?

[MadaraUchiha-314]

If a message has no role how do you distinguish it from other message types?

Are you talking about message.type or message.role

In langgraph provided message interfaces like HumanMessage, ChatMessage and BaseMessage, they all have the attribute called type

role is not present in most of them